1. Field potentials and extracellular potassium concentration, [K+]o, were recorded from the rat cerebellar cortex using ion-selective micro-electrodes, following micro-stimulation of the cerebellar surface. The compound action potential of the parallel fibres (p.f.s) showed changes indicative of a supernormal period (s.n.p) when conditioned by a previous p.f. volley, and was studied in relation to [K+]o. 2. Repetitive stimulation of the p.f.s (greater than 10 Hz) elicited an alternation in p.f. excitability from supernormality to subnormality simultaneous to a steady increase in [K+]o. 3. Superfusion with various levels of K+ led to changes in the p.f. conduction properties. Small increases in [K+]o above the resting 3.0 mM level led to an increase in p.f. conduction velocity while greater increases led to conduction slowing and eventually block. 4. Repetitive activation of a row of p.f.s elicited increases in [K+]o in the vicinity of neighbouring non-activated fibres. These fibres displayed an increase in excitability that was quantitatively related to [K+]o. 5. After introduction of 4-aminopyridine (4-AP; 100 microM) into the superfusate, a single stimulus would elicit relatively large (up to 15 mM) increases in [K+]o around neighbouring non-activated p.f.s. The excitability of the adjacent non-activated fibres was either increased or decreased, and was quantitatively related to [K+]o. 6. Strophanthidin application (15 microM) led to a slow and continuous increase in [K+]o. The excitability of the p.f.s initially increased as [K+]o increased, but subsequently decreased, eventually resulting in conduction block. 7. These experiments are consistent with the hypothesis that small increases in [K+]o may elicit an increase in p.f. excitability while greater increases lead to a decrease in p.f. excitability.